Jet lag occurs because your internal body's clock, also called the circadian rhythm, is still synced to your original time zone, instead of the new time zone at your arrival destination.
The circadian rhythm is a roughly 24-hour physiological cycle happening in your body every day. There are clear patterns of brain wave activity, hormone production, cell regeneration and other biological activities linked to this daily cycle.
When you travel across time zones this natural daily cycle is interrupted and you experience jet lag. The more time zones crossed, the longer the jet lag will last.
Your internal body clock will in general use one day per timezone offset to fully adjust to the new destination time. For example, if you crossed six time zones, the body will typically adjust to this time change in four to six days.